Top loaded monopole antenna with guy wire line tightener



April 23, 1968 M. A. LARSON ETAL 3,380,061

TOY LOADED MONOPOLE ANTENNA WITH GUY WIRE LINE TIGHTENER Filed March 4, 1965 2 Sheets-Sheet l //VVE/V7'OR5 MA/ew/v A. LARSON ROBE/er L. TAM/ale April 23, 1968 M. A. LARSON ETAL 3,380,061

TH GUY WIRE LINE TIGHTENER TOP LOADED MONOPOLE ANTENNA WI 2 Sheets-Sheet Filed March 4, 1965 TOWER END OF CONDUCTOR WNSULATOR END oF CONDUCTOR l O O O O IPOZMJ ICZS Hun,

S/A RELATNE DWTANCE FROM'UP OF TOP- LOAD R 3w W. mi N R A O E AT V L W W A 7 New W5 M A MP7 5 6 United States Patent 3,380,061 TOP LOADED MQNQPGLE ANTENNA WITH GUY WIRE LINE TEGHTENER Marvin A. Larson, Redwood City, and Robert L. Tanner,

Merrie Park, (Iaiif, assignors to Control Data Corporation, Minneapolis, Minn, a corporation of Minnesota Filed Mar. 4, 1965, Ser. No. 437,120 9 flairns. (Cl. 343-874) ABSQT 0F THE DISCLGSURE Means are provided for maintaining taut the guy wires of a top loaded monopole antenna by coupling a concentrated weight by means of a shackle to the end of top loaded conductor which is near the bottom, with provision being made to damp out or eliminate any lateral swinging of the weight. Furthermore, the lower end of each top loaded conductor is bent back upon itself for a portion of its length to reduce the field gradient at the lower end of the top loaded conductor.

This invention relates to monopole antennas having top load conductors and more particularly, to improvements therein.

In monopole antennas, used as transmitting antennas at low frequencies, it is common practice to attach to the top of the antenna tower several radially disposed condoctors. These extend radially and are anchored to the ground at a distance from the base of the tower. The lower ends of these conductors are attached to a series of insulators, with the first insulator usually located at the mid span or somewhat below, and the last being connected to an anchor or wire or cable. The purpose of such conductors, which are usually referred to either as top loads or as umbrella conductors, is to increase the effective height and capacitance of the antenna and thereby to increase its frequency bandwidth, its efficiency and its power radiating capability.

It has been found that the maximum electrical benefit may be obtained from the top load conductors if they are stretched taut enough to form a nearly straight line between the top of the tower and their anchor points. When in such position, the centroid of the charge existing on the top load is as high as possible. (It is known in the are that one way of defining the effective height of a low frequency antenna is as the height of the charge centroid.) When the top load conductors are held taut, however, severe structural probiems are introduced. A wind which blows crosswise on a taut line or cable causes a transverse force to be induced in the cable which is transformed into a greatly magnified axial force thereby. In the case of multiple top load conductors attached to an antenna tower, the resultant of all these forces is a large force acting axially on the tower, which can cause structural failure of the tower or rupture of the conductors themselves.

A similar situation exists in the case of ice build up on the conductors. The added weight of the ice on t-aut conductors is transformed again with high magnification into axial stresses in the conductors.

To avoid the possibly disastrous consequences that can ensue under wind or ice load conditions with taut top load conductors, it is customary to allow the conductor to sag very substantially. However, this lowers the charge centroid of the antenna and thereby degrades its electrical characteristics.

Heretofore, to help overcome the effects of sag, counterweighting was employed. To counterweight, the ground end of the top load cable is passed over a pulley which is attached to the top of a post or frame installed at the CFI 3,380,061 Patented Apr. 23, 1968 ice ground anchoring point of the cable. A weight is attached to the end of the cable which can then slide up and down on guides or rails fastened to the frame. With such an arrangement, the top load conductor is held taut by the weight in the absence of wind. When a wind load or an ice load is applied, the cable is allowed to deflect and does not develop the high axial stresses which would otherwise occur. However, such top load counterweighting schemes have the disadvantage of being expensive both to install and to maintain functioning freely movable, as is required for proper response to the adverse conditions which have been mentioned. For this reason,

this technique is seldom used.

Accordingly, an object of this invention is to provide a novel and simple arrangement for maintaining a top load conductor taut in a manner so that it will not be unduly stressed by the effects of wind and ice.

Another object of the present invention is the provision of an inexpensive arrangement for maintaining top load conductors taut while preventing them from having high axial tension in response to wind or ice loads.

Yet another object of the present invention is the provision of a maintenance-free arrangement for keeping the top load conductors of a monopole antenna taut while preventing undue tension being induced in said conductors in response to wind or ice loads.

Another consideration in the design of top load conductors is the high electric field or potential gradient that exists at the surfaces of these conductors. When a voltage is applied to the antenna the charge which flows onto the antenna tends to concentrate strongly at isolated extremities thereof, producing high field gradients. In this respect, the tips of the top load conductors are the most vulnerable areas on the antenna. Corona discharges occur at these tips because of the high field concentrations which are set up there by the voltage which is applied to the antenna. Since the power radiating capability of the antenna is proportional to the square of the voltage that can be applied to it, and since this voltage in turn is limited by the corona discharges which occur at the tips of the top load conductors, if a way can be found for reducing the field concentrations, then the voltage applied to the antenna can be raised whereby the power radiating capability of the antenna is raised.

Since the voltage gradient produced by a given applied voltage is inversely proportional to the radius of the top load conductors, by increasing the diameter of the top load conductors the voltage rating can be increased. This is the method usually employed; however it does have a number of disadvantages. Larger diameter conductors are more expensive since an increase in the conductors diameter by a factor of 2 increases the material in the conductor by a factor of 4. In addition, larger conductors are both heavy and subject to greater wind loads because of the greater area they present to the wind. Both of these factors increase the axial stresses of the conductors requiring larger and more expensive insulators and ground anchors as well as requiring sturdier structural design in the supporting tower. It will be appreciated that all of these considerations indicate an increase in cost for the construction of an antenna of a specified electrical performance.

Accordingly, still another object of the present invention is the provision of a structural arrangement for the top load conductors of a top loaded antenna which reduces the field concentrations existing at the tips of the top load conductors whereby the voltage ratings of the antenna is increased.

Still another object of the present invention is the provision of a unique arrangement for the top load conductors of an antenna which improves its power handling capability and performance without increasing its costs.

Yet another object of the present invention is the provision of a novel arrangement for top load conductors of a top loaded antenna which does not cause an increase in the stress induced in these conductors due to wind or ice loads.

The foregoing and other objects of the present invention are achieved by connecting a concentrated weight by means of a shackle to the end of the top loaded conductor near its bottom end, with provision being made to damp out or eliminate any lateral swinging of the weight. Furthermore, the lower end of each top loaded conductor is bent back upon itself for about one-third of its length whereby the field gradient at the end of the top load condoctor is considerably reduced. As previously indicated, this enables the voltage which is applied to the antenna to be increased.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic drawing of a monopole antenna illustrating the difference between a top loaded conductor with sag and a top loaded conductor wherein the sag has been substantially eliminated in accordance with this invention;

FIGURES 2, 3 and 4 illustrate different arrangements in accordance with this invention, for preventing undue I swinging of the top loaded conductors;

FIGURE 5 is a chart illustrating an actual plot of the relative axial liue-charge density as a function of position along the top load conductor;

FIGURE 6 shows an arrangement of a top load conductor for redu-cig the potential gradient at the insulator end of a top load conductor; and

FIGURE 7 is a drawing illustrating the manner in which the insulator end of a top load conductor may be folded back on itself in accordance with this invention.

Referring now to FIGURE 1, there may be seen schematically a monopole antenna which includes the tower 10 supported vertically with respect to the ground by guy wires, not shown. The tower 10 will have a plurality of top load conductors, exemplified by top load conductors 12, 14. These top load conductors are attached at one end to the top of the tower 10. The other end is connected to ground through a series string of insulators, exemplified by insulator 20. The series insulator string reduces the potential gradient across any single insulator. As was indicated previously, in order to prevent the stress which can occur when a top load conductor is stretched taut, the top load conductor is allowed to sag in the manner represented by the top load conductor 12. This places the centroid of the charge lower than happens when the top load conductor is held in the manner represented by top load conductor 14. Top load conductor 14 is held taut and higher than top load conductor 12, in accordance with this invention, by using a weight 22 which is attached to the wire connected through the strain insulators to the end of the top load conductor 14. As may be seen in more detail in FIGURE 2, the weight 22 is attached to the Wire 24 by means of a shackle 26, which holds the weight in place. The other end of the wire 24 may be connected to ground in the usual manner. The weight which is applied near the bottom end of the wire 24 has the effect of pulling the top load conductor 14 taut, thereby increasing its height substantially, whereby the efficiency of the antenna and its bandwidth are increased substantially. However, when a wind load, or ice load, occurs on the conductor, there is no catastrophic increase in tension. Instead the conductor deflects, lifting the weight with substantially no increase in tension on the conductor.

A potential problem that may arise by the simple connection of the weight 22 to the top load conductor is that the weight and top load conductor system may comprise a long period pendulum which, when set in motion by the winds or other forces, can continue to swing for an undesirably long time. This may be prevented in the manner shown, for example, in either FIGURE 2 or FIGURE 3. In FIGURE 2, the length of cable between the shackle supporting the Weight and the anchor can be replaced by two cables 30, 32 whose points of attachment respectively 34, 36, at ground are spread laterally. This arrangement has the eflect of stabilizing the weight and preventing swinging. Alternatively, as shown in FIGURE 3, a length of chain 38 may be attached to the bottom of the weight 22, which is long enough to drag on the ground. The dragging chain will absorb any energy applied to the cable and weight and will quickly dampen any oscillation.

Where the space available for antenna installation is too limited to allow an adequate length for the attachment cable 24, shown in FIGURE 1, an arrangement may be employed such as is shown in FIGURE 4. Here, the attachment cable 40 to which the weight 22 is attached has one end fastened to the insulator string 20, which in turn is connected to the lower end of the top load conductor 14, and the other end of the attachment cable is attached to a short post 42. The post, of course, is suitably anchored. Typically, if the space devoted to the attachment cable, as shown in FIGURE 1, is fifty feet, this distance may be reduced to ten feet and the equivalent structural advantages may be obtained by fastening each one of the attachment cables to a different post, wherein each of the posts may be ten feet high.

FIGURE 5 shows a plot of the relative axial linecharge density as a function of position along a top load conductor. It should be noted that the abcissa of the plot is a logarithmic scale so that the portion near the tip of the conductor is greatly expanded. The relative potential gradient at the surface of the conductor is proportional to the charge-density. As may be seen by the curve, the charge-density at the end of the top load conductor which is farthest away from the tower is quite high. Since the power radiating capability of the antenna is proportional to the square of the voltage that can be applied to it, is is desirable to prevent such eifects as corona discharge occurring at the tips of the top loaded conductors due to high field concentrations existing there, from serving as a limitation upon the voltage which can be applied to the antenna.

The voltage gradient produced by a given applied voltage is inversely proportional to the radius of the top load conductors. Consequently, the voltage gradient of the antenna may be reduced, in the manner previously indicated as being practiced in the prior art, by increasing the diameter of the top load antennas. Further, as previously indicated, with an increase in the diameter of the top load conductor, not only are the costs of these top load conductors increased but the added weight and wind load occasioned by the increase size serves to increase the load which is applied to the tower to an extent that larger and more expensive insulators, ground anchors, and a sturdier structural design for the tower must be established.

Inspecting FIGURE 5, it is evident that while the maximum charge-density, and consequently the maximum field occurs near the the tip of the top load conductor, it diminishes away from the tip so that at approximately one-third of the distance from the conductors tip to the tower top the axial charge-density has dropped to a value which is one-half of its value at the tip. In accordance with this invention, it is possible to apply a higher voltage to the antenna while still using the same relatively small diameter cables for the top load conductors with which only a relatively lower voltage was possible hereto fore. This is achieved in the manner shown in FIGURE 6 wherein the top load conductor 44 is made sufficiently long so that the lower end may be doubled back on itself to a distance of approximately one-third of the distance to the tower. In other words, the top load conductor is made one and one third of the length required to reach the nearest insulator. The doubled back end consists of two parts 44A, 44B.

As shown in FIGURE 7, the doubled back parts 44A, 44B are spaced apart from one another a distance equal to several diameters of the cable, preferably ten or more, by a suitable corona protected spacing member 46. This spacing member may be a ceramic insulator which is in the form of a hollow cylinder. The doubled back members 44A, 44B pass through the opening in the hollow cylinder. The attachment cable 48 passes around the periphery of the cylinder. One or more corona shields 56, 52,, as is the common practice, may be attached to the end of the double backed portion of the top loaded conductor 44. However, with the doubled back arrangement shown, the corona shields may be omitted if desired with a resulting reduction in voltage rating of only about The total axial charge is increased only very slightly when the top load conductor is doubled as shown. The charge now divides equally between the two parts of the doubled conductor, however, and if the individual parts are spaced a distance of several diameters, in the manner shown in FIGURE 7, the charge distributes very nearly uniformly over the surfaces of the two portions of the top load conductor. Since the surface area is now twice as great as with a single conductor, the surface charge density, and consequently the surface electrical field, is only about half as great as with a single conductor. The doulbled portion of the conductor is carried back up the top load for approximately one-third of its total length. Here, where the spacing between the two ends 44A, 44B becomes less, the effect in reducing surface fields also becomes less. However, the natural drop off of the field, as indicated in FIGURE 5, is to approximately half the value at the top. Consequently, the field at the surface of the single conductor will be well below the corona threshhold. Furthermore, the corona protection provided by the double back arrangement described herein is adequate.

There has accordingly been described and shown herein a novel and useful arrangement for increasing the efiiciency and power handling capability of a monopole antenlna through the technique of raising the centroid of charge, as well as enabling the antenna to have higher voltages applied to its without corona discharge occurring. It will be apparent to those skilled in the art that already constructed monopole antennas may be modified in accordance with this invention by attaching a weight to each attachment cable to which a top load conductor is attached and also be either replacing each top load conductor with one having the one-third additional length or by attaching an additional length to each of the top load conductors already in place. These modifications therefore are to be considered within the spirit and scope of the claims herein.

What is claimed is:

1. In a monopole antenna of the type wherein a tower which is vertical to the earth and has a plurality of top load conductors extending radially from the top end thereof, each of said top load conductors having one end coupled to the top of said tower and the other end connected through insulator means and an attachment cable to ground, said improvement comprising a weight attached to said attachment cable to be held thereby above the ground, the mass of said weight serving to hold said top load conductor taut and higher than without said weight, and means attached to said weight to minimize the swinging thereof.

2. The improvement as recited in claim 1 wherein said means to minimize the swinging of said weight comprises an additional attachment cable, means connecting one end of said additional attachment cable to the attachment cable to which said weight is attached, and means connecting the other end of said additional attachment cable to ground at a location spaced from the location at which said attachment cable is attached.

3. The improvement as recited in claim 1 wherein said means for preventing said weight from swinging comprises a second weight, means for attaching said second weight to said weight with an attachment long enough to permit said second weight to be dragged along the ground.

4. In a monopole antenna of the type having a vertically supported tower and a plurality of top load conductors radially extending outwardly from said tower, each one of said top load conductors having one end attached to the top end of said tower and means including insulator means and a ground attachment cable for attaching the other end of a top load conductor to ground, the improvement comprising for each top load conductor a post supported in the ground, means for attaching the end of the ground attachment cable to said post, a weight, and means for connecting said weight to said ground attachment cable to increase the effective height of said top load conductor and thereby the centroid of charge of said antenna.

5. A monopole antenna comprising a tower which is vertical with respect to the surface of the earth, a plurality of top load conductors extending radially outward from the top of said tower, means connecting one end of each of said top load conductors to the top of said tower, a plurality of insulator means for connecting the other end of each of said top load conductors to ground, each of said top load conductors having a length which is substantially greater than the length required to reach one of said insulator means connecting it to ground, said portion of the length of each of said top load conductors which exceeds the length required to reach said insulator means being folded back on the remainder of said top load conductor, and means for spacing said folded back portion of said top load conductor at the lower end thereof a distance on the order of a plurality of diameters of said top load conductor.

6. A monopole antenna as recited in claim 5 wherein said portion of the length of each of said top load conductors which exceeds the length required to reach said insulator means is on the order of one-third of the length of said top load conductor.

7. In a monopole antenna of the type having a vertically supported tower and a plurality of top load conductors extending radially outward from the top end of the tower, each having one end attached to the top of said tower and having insulator means attaching the other end to ground, the improvement comprising for each top load conductor an additional conductor length extending back on said top load conductor part way to said tower from the end attached to said insulator means, said additional length being spaced from said top load conductor at the end where it doubles back a distance on the order of a plurality of diameters of said conductor.

8. In a monopole antenna of the type wherein a tower which is vertical to the earth and has a plurality of top load conductors extending radially from the top end thereof, each of said top load conductors having one end coupled to the top of said tower and the other end connected through insulator means and an attachment to ground, said improvement comprising a weight attached to said attachment cable to be held thereby above the ground, the mass of said weight serving to hold said top load conductor taut and higher than without said weight, means attached to said weight to minimize the swinging thereof, and comprising for each top load conductor an additional conductor length extending back on said top load conductor part 'way to said tower from the end attached to said insulator means, said additional length being spaced from said top load conductor at the end where it doubles back a distance on the order of a plurality of diameters of said conductor.

9. In a monopole antenna of the type having a vertically supported tower, a plurality of top load conductors extending radially outward from the top of the tower and having each of their other ends connected to ground through an insulator means connected in series with a ground attachment cable, the improvement comprising in place of each of said top load conductors a top load conductor having a length which is on the order of onethird greater than the length required to extend from the top of the tower to said insulator means, said additional length being folded back on said top load conductor, and a weight attached to each of said ground attachment cables to hold each of said top load conductors taut.

References Cited UNITED STATES PATENTS FOREIGN PATENTS Great Britain.

ELI LIEBERMAN, Primary Examiner. 

